I'm a week late 🎃 but do any #CFD folks want to see a ghost?

One versatile technique for compressible flow is to implement "characteristic" boundary conditions by using a #RiemannSolver with a freestream solution outside. This nice paper discusses the technique and reports that HLL works as well as the more accurate solvers. But HLL doesn't represent the contact wave and fun things happen after the cold bubble reaches the outflow. https://doi.org/10.2514/6.2014-2923